HTLV-I-encoded Tax protein, which appears responsible for the pathogenesis of the virus, is critical to its life cycle, activating viral transcription through interaction with the cellular transcription factor CREB. The precise molecular mechanism by which Tax mediates strong transcriptional activation through CREB remains incompletely understood. Recent evidence suggests that Tax, through interaction with CREB, promotes the high affinity recruitment of the coactivator CBP to the viral promoter. Tax interacts with a 200 amino acid region of CBP called the KIX domain. Tax recruitment of KIX requires CREB and the GC-rich nucleotides that flank the CREB binding site. Tax recruits KIX to protein-DNA complexes containing phosphorylated or unphosphorylated CREB. Surprisingly, Tax also recruits KIX to the 73 amino acid basic leucine-zipper domain of CREB. Evidence indicates that CBP is a potent cofactor for Tax-mediated transactivation, as the protein strongly augments Tax stimulation of the HTLV-I promoter. These data support a model where Tax interacts with CREB and the viral CRE, where it serves as a high affinity binding site for CBP, anchoring the coactivator to the HTLV-I promoter. The goal of this proposal is to characterize the protein-protein contacts between Tax and the KIX domain of CBP, and to dissect the molecular interactions between Tax and the GC-rich nucleotide sequences which immediately flank the viral CRE core. These studies should provide a more fundamental understanding of the mechanism of Tax transcriptional activation of HTLV-I.